1. Field of the Invention
The present invention relates to a device for coding a picture signal by compression and a device for decoding a compressed picture signal by expansion. More particularly, the present invention is concerned with a picture signal compression coding device which maintains the amount of compressed data constant, eliminates distortions in the event of reproduction, which does not use elements operating at high speed, an expansion decoding device for decoding so coded data, and a system including such coding and decoding devices.
2. Description of the Related Art
Digital picture data representative of a picture picked up by an electronic still camera, for example, are stored in a memory. Various kinds of compression coding schemes have been proposed to reduce the amount of such digital picture data and thereby the required memory capacity. Among them, a two-dimensional orthogonal transform coding scheme is extensively used because the coding scheme codes data by a large compression ratio and because the coding scheme allows a minimum of picture distortions particular to coding to occur.
Two-dimensional orthogonal transform coding is such that picture data representative of a single picture are divided into a plurality of blocks, and the picture data are subjected to two-dimensional orthogonal transform block by block. The picture data undergoes orthogonal transform, i.e., transform coefficients are compared with a predetermined threshold so as to discard those transform coefficients which are lower than the threshold. The transform coefficients lower than the threshold are treated as data zero thereafter. The remaining data are divided by a predetermined quantizing step value, or normalizing coefficient, and thereby quantized or normalized by a step size. By this kind of procedure, the values of transform coefficients, i.e., the dynamic range of amplitudes is suppressed.
The two-dimensional orthogonal transform coding procedure stated above has some problems left unsolved. Specifically, coding picture data by applying a predetermined normalizing coefficient to transform coefficients results in the amount of data which is different from one coded picture data to another. Such coded picture data cannot be written in a memory whose capacity is limited without resorting to troublesome processing. More specifically, when a predetermined normalizing coefficient is used, picture data containing many high frequency components will have a large amount of data when coded while picture data containing many low frequency components will have a small amount of data when coded. The coded data of the picture data in which high frequency components are predominant sometimes amounts to five to ten times of the coded data of the picture data in which low frequency components are predominant.
When a particular kind of picture is divided into blocks as stated previously, there occur two different groups of blocks, i.e., a group of blocks containing many high frequency components and a group of blocks containing many low frequency components. Then, despite that the picture as a whole has more high frequency components than low frequency components, for example, some blocks will contain more low frequency components than high frequency components. In this condition, applying a large normalizing coefficient to all the blocks which constitute the picture causes distortions in the normalized picture data of the blocks which contain many low frequency components. This would degrade the quality of a reproduced picture.
Further, the two-dimensional orthogonal transform coding sequence needs division which uses a normalizing coefficient. The division is not practicable without using a high-speed and accurate divider and, therefore, without increasing the scale of the device.